1. Field of the Invention
The present invention relates to an image processing technique for generating data by applying compression processing to an image obtained by reading a paper original, etc.
2. Description of the Related Art
The digitization of documents has been advanced in recent years by the spread of scanners. However, when a digitized document in a full-color bitmap format is to be stored, approximately 24 MB are required at 300 dpi in the case, for example, of an A4-sized original. Thus a very large amount storage space is required. Further, such a large quantity of data is not well-suited to electronic transmission.
In general, therefore, compression processing is applied to the full-color image data. The JPEG scheme is well-known as one such compression scheme. The JPEG scheme is highly effective in compressing natural images such as a photograph and affords a high image quality. On the other hand, when a high-frequency portion such as a text image is subjected to JPEG compression, image degradation referred to as “mosquito noise” occurs and the compression rate declines as well. Documents in common use often contain both text and images on the same page and it is difficult to obtain both good image quality and a high compression rate using JPEG compression.
One method that has been proposed in order to solve this problem includes segmentation of an image into regions, applications of JPEG compression to a background portion of the image from which text regions have been removed and application of MMR compression to text regions having color information. When data thus compressed is decompressed, text images obtained by decompressing the MMR-compressed part of the image are placed upon a background image obtained by decompressing the JPEG-compressed image. That is, with regard to white portions among the text images, the background image is allowed to pass and the representative character color is placed upon the black portions. For example, an image processing apparatus disclosed in the specification of Japanese Patent Application Laid-Open No. 2002-077633 creates output data from an image, which has been obtained by scanning in a paper document, by applying lossless compression to text regions through binarization of these regions without a decline in resolution, and applying highly aggressive JPEG compression to the background image with lowered resolution. As a result, a large scale reduction in size can be achieved while maintaining the color information and without sacrificing text readability, and it is possible to obtain a digitized document ideal for transmission or storage.
Meanwhile, due to the spread of computers, operations such as the creation and editing of documents using document editing applications like word processors is now commonplace. There is a growing demand not only for the ability to view documents stored in electronic form as described above, but also for the ability to use images in portions of one document by inserting the images into other documents. However, since a document that has been put into electronic form by scanning is compressed in order to improve storage efficiency as mentioned above, degradation of the image by this compression becomes a trouble in cases where the image is utilized in the creation of another document.
In a document that has been put into electronic form by the image processing apparatus disclosed in the specification of Japanese Patent Application Laid-Open No. 2002-077633, if the data is from a text region, because it undergoes lossless compression and is stored without a decrease in resolution, it is possible to extract and utilize images from this region without concern over image degradation. However, since regions other than the text region undergo high-compression processing as a background region, the problem of image degradation at the time of utilization cannot be avoided.
Because of greater opportunities to create documents electronically, there are many cases in which the original of a paper document that is to be scanned has been created by one's own computer or by a computer on a LAN. There are also many cases in which the original data of image areas such as photographs and drawings contained in such a document has been stored somewhere. Accordingly, it may be construed that there are many instances in which the original data can be obtained if an image database connected to a LAN, for example, is searched. If an image region such as a photograph or drawing is utilized in such an environment, it is possible to create another document having a high image quality by utilizing the original image data rather than compression-degraded image data contained in a document digitized earlier. In such case the original image data is obtained by searching the image database using as a key the image data representing the photograph or drawing, etc., of the digitized document.
There is an arrangement in which a feature amount that is for the purpose of searching image data is registered by a human being through use of a keyword or the like, and an arrangement in which the feature amount is extracted from image data by computer. The latter is referred to as an image feature amount, which may be a color histogram, a color structure, an edge histogram or texture, etc., extracted from an image.
In addition to image feature amounts, a description method has been standardized by MPEG-7 Visual (ISO/IEC 15938-3), etc. For example, in an image database having image feature amounts that are in compliance with this standard, the original image data can be retrieved by acquiring an image feature amount complying with the same standard.
However, when searching for an image feature amount, in an image database, from heavily compressed data such as digitized document data, a problem arises in that a precise search cannot be conducted because of the degradation of the images due to the effects of compression. For example, in cases where a search is conducted using an image feature amount such as an edge histogram, it is difficult to obtain a given feature amount, which correctly expresses the feature amount of the original image, from an image that has undergone lossy compression at a high compression rate. The reason for this is that in the case of an image whose resolution has been lowered in order to reduce its size or an image that has been subjected to JPEG compression at a high compression rate, some high-frequency information such as that of an edge is lost.